The commercially available and industrially important cellulose acetates are almost all produced by reaction of cellulose and acetic anhydride using strong acids as catalysts and acetic acid as a solvent. Compounds generally used as catalysts for cellulose acetylation include sulfuric acid, perchloric acid, and sulfonic acids. Sulfuric acid, the most commonly used catalyst, is believed to be the best catalyst for the commercial manufacture of cellulose acetates in an acetic acid system. Other catalysts are precluded from commercial applications because they are not effective, impractical, or dangerous to practice on an industrial scale. See, Malm, C. J., Hiatt, G. D., Cellulose and Cellulose Derivatives, Part II of High Polymers, Ott, E., Spurlin, H. M., Graffin, M. W. eds., Vol. 5, 2d. ed., Interscience Publishers, New York, 1954.
A typical industrial cellulose acetate process using sulfuric acid as the catalyst in the so-called acetic acid system usually comprises at least two steps. First, the cellulose raw materials are activated or pretreated by mixing the raw material with water, aqueous acetic acid, and acetic acid, or acetic acid containing small amount of sulfuric acid. To be useful in an acetic acid process, the cellulose raw material generally has a high .alpha.-cellulose content and a relatively high molecular weight. To accomplish the acetylation, the activated cellulose is reacted with a mixture of prechilled acetic anhydride, acetic acid and sulfuric acid to produce primary cellulose acetate. The primary cellulose acetate produced in the acetylation step, also called cellulose triacetate, may be subjected to hydrolysis (or ripening) to produce cellulose acetate of any desired degree of acetyl content.
The typical industrial cellulose acetate process suffers from one or more of several disadvantages. A typical industrial process requires very high quality cellulose raw materials having a high .alpha.-cellulose content. These high quality celluloses are usually required to meet the desired end-use applications of the cellulose acetate product. However, the acetylation step, particularly when catalyzed with sulfuric acid, can degrade the cellulose polymer resulting in cellulose acetate products having low intrinsic viscosity (IV) and low molecular weight (MW). Low .alpha.-cellulose raw materials, for this reason, generally cannot be used in typical industrial processes.
The acetylation step in a typical industrial cellulose acetate process is highly exothermic producing a large quantity of heat. Therefore, not only must the liquid chemicals consisting of acetic acid and acetic anhydride be cooled before acetylation, but the reactor must also generally be cooled externally to regulate the reaction. This cooling is necessary to obtain cellulose acetate with the desired degree of polymerization. Accordingly, typical industrial cellulose acetate processes often require a large quantity of refrigeration to cool and regulate the reaction temperature during the acetylation step. This increases the cost of production.
The typical industrial process also requires a certain amount of a sulfuric acid catalyst (generally 6% to 20% based on the weight of dry cellulose) to uniformly acetylate the cellulose and produce cellulose acetate with good solubility in the acetic acid medium. To recover and process the desired cellulose acetate, the large amount of sulfuric acid catalyst must be neutralized during or after acetylation or as a step in a later hydrolysis processes. Having to neutralize the sulfuric acid increases production costs and may result in subsequent production difficulty such as plugging of the processing lines by insoluble sulfate salts. Higher sulfuric acid concentrations during acetylation generally result in a cellulose acetate product having higher chemically combined sulfuric acid in the cellulose acetate product. Cellulose acetate with high levels of chemically combined sulfuric acid is not heat stable.
Various processes have been reported to overcome some of the disadvantages of a conventional cellulose acetate production process. U.S. Pat. No. 2,923,706, corresponding to Canadian Patent No. 565,099, reports using small amounts, less than 1% by weight of dry cellulose, of sulfuric acid as catalyst while performing the cellulose esterification at temperatures above 50.degree. C. The process, however, requires that the sulfuric acid be uniformly distributed throughout the cellulose be means of a special pretreatment process.
U.S. Pat. No. 3,767,642 reports a process for producing secondary cellulose acetate from low cost wood pulp with low .alpha.-cellulose content. According to this patent, the secondary cellulose acetate is produced. from primary cellulose acetate by a high-temperature hydrolysis process at a hydrolysis temperature ranging from 125.degree. C. to 170.degree. C.
U.S. Pat. No. 4,439,605 also reports a process to produce secondary cellulose acetate using high-temperature acetylation and hydrolysis steps. The primary cellulose acetate for this process is produced by acetylating cellulose at a temperature from 50.degree. C. to 85.degree. C. with a relatively low sulfuric acid catalyst concentration, ranging from 0.5 to 5.0 weight parts per 100 weight parts dry cellulose . The process fully neutralizes the acid catalyst before introducing steam into the reaction mixture to elevate the system's temperature to between 125.degree. C. and 175.degree. C. Maintaining this temperature for 30 minutes to 6 hours hydrolyzes the primary acetate to cellulose diacetate. Reportedly, this process may also be used to produce cellulose acetate from low-grade pulps.
A process for producing cellulose acetate from a low .alpha.-cellulose raw material by selectively depolymerizing the impurity in the hemicellulose, in particular glucomannan, in the raw material has been reported in European Patent Application No. 0 626 391 A1. According to this report, the depolymerizing is accelerated by increasing the amount of acetylation catalyst, in particular sulfuric acid, in the pretreatment. Acetylation is then conducted while controlling the reaction temperature in the range of 50.degree. C. to 85.degree. C. Hydrolysis is effected at a temperature ranging from 125.degree. C. to 175.degree. C., after neutralizing the sulfuric acid catalyst.
Another approach to the synthesis of cellulose acetate has been reported in U.S. Pat. No. 2,585,516. As described in this patent, the cellulose is impregnated with a mixture of a carboxylic acid derivative and a compound of an inorganic sulfoxy acid and esterified with aliphatic acid anhydrides, such as acetic anhydride, at temperatures above 90.degree. C. The carboxylic acid compounds used are amides or carboxylic acid salts formed with ammonia or an amine having at least one amino hydrogen. The inorganic sulfoxy compounds include sulfuric acid, its partial amides and esters, sulfurous acid, thiosulfuric acid, tetrathionic acid, persulfuric acid as well as their salts with ammonia or an amine having at least ,one amino hydrogen.
Takahashi and Takahashi, (Kobunshi Kagaku, Vol. 27, No. 302, p. 394, 1970), report a similar process for synthesizing cellulose acetate. The process involves immersing cotton linters as a cellulose raw material in an aqueous acetic acid solution of various sulfates containing metallic or ammonium ions. After the excess imbibition acid solution was removed, the raw material is dried before acetylation. The resultant cellulose having a catalyst concentration of 2%, is then acetylated at 98.degree. C. Cotton linters impregnated with catalysts such as Cd--, Zn--, or Al-sulfate or Fe-alum were reportedly acetylated to triacetyl cellulose smoothly within 8 minutes and those impregnated with (NH.sub.4).sub.2 SO.sub.4, (NH.sub.4)HSO.sub.4, (NH.sub.4).sub.2 S.sub.2 O.sub.8 or Mohl's salt were similarly acetylated within 25 minutes. For the sulfates containing Fe or NH.sub.4 cations, the reaction became faster as the salts contained a larger amount of SO.sub.4.sup.2-. It was also found that the degree of polymerization of the acetates was markedly decreased due to the high reaction temperature.
European Patent Application No. 0 638 244 A1 reports a process to acetylate cellulose using an adduct of sulfuric acid and N,N-dimethyl acetamide as a catalyst. The acetylation is conducted with acetic acid solvent at a temperature of 10.degree. C. to 90.degree. C. with 1.5% to 30% catalyst by weight of catalyst adduct. A disadvantage in this process, however, relates to its by-products. The N,N-dialkylamides, ammonia by-products, and amine by-products resulting from the reaction with acetic anhydride during acetylation are generally unacceptable for industrial practice due to their posssible toxicity.
Cellulose acetate production processes, such as those discussed above, typically acetylate cellulose at high temperature with a sulfuric acid catalyst, hydrolyze at still higher temperatures, or both acetylate and hydrolyze at very high temperatures. Certain disadvantages exist in such processes. For example, high temperature processes generally produce cellulose acetate products having poor solubility in organic solvents, such as acetone, or with low whiteness or both. Excessive reaction temperatures also tend to degrade the cellulose polymeric structure yielding cellulose acetate products with low intrinsic viscosity and corresponding low molecular weight.